refactor(pipelock): start/stop become methods on PipelockProxy

ProxyPlan -> PipelockProxyPlan, with two additional fields populated
in launch: internal_network, egress_network (default ""). prepare
fills yaml_path + slug; launch uses dataclasses.replace to populate
the networks before calling start.

pipelock_start -> PipelockProxy.start(plan). Reads yaml_path, slug,
internal_network, egress_network off the plan. Returns the resolved
container name.

pipelock_stop -> PipelockProxy.stop(proxy_target). Takes the resolved
container name directly (the value that start returned); no longer
needs to know about slugs or naming conventions.

Backend launch passes the running container name (state["pipelock"])
to stop. Test for stop's idempotency uses pipelock_container_name to
construct the proxy_target.

tests/test_pipelock_sidecar_smoke.py

@@ -38,7 +38,7 @@ class TestPipelockSidecarSmoke(unittest.TestCase):
     )
     def test_smoke(self):
         yaml_path = self.work_dir / "pipelock.yaml"
-        PipelockProxy().prepare(fixture_minimal(), "dev", yaml_path)
+        PipelockProxy().prepare(fixture_minimal(), "dev", "demo", yaml_path)
 
         create = subprocess.run(
             [